Homework Helpers: Physics

8 Waves and Light

Lesson 8–4: Light

Now that you know some of the properties of waves, you can think about how these properties are exhibited by light. This is just what earlier scientists did during the particle-wave debate. If you want to argue that light travels in waves, you need to show that light exhibits wave-like properties.

Anyone who has looked in a mirror or the surface of a still body of water must accept the fact that light exhibits the property called reflection. As light strikes a surface, some portion of it will be turned back. Unfortunately, this doesn’t really prove that light is made up of waves, because particles exhibit reflection as well. Understanding this is what allows billiards players to make bank shots.

Light can also be refracted when it passes obliquely (at an angle less than 90°) into a new medium. However, it is easy to imagine how the path of a particle might get refracted as well. For example, if a golf cart that was driving on a paved pathway passed onto the wet grass of a golf course at a 45° angle, the tires on one side of the cart might slip first, causing the cart to turn slightly. So, the fact that light refracts was not considered conclusive evidence to support a wave-theory of light.

For a time, the fact that light did not seem to exhibit diffraction was used as evidence to argue against the wave-theory of light. If your teacher were to duck behind her desk and keep talking, you would still hear her, because sound waves diffract, or spread out, beyond the barrier of the desk. You would not be able to see her, however, so light doesn’t seem to diffract in this case. However, it was eventually shown that light does diffract if you use an opening or barrier that is sufficiently small. If you block a light source with a very small object, the object can be shown to cast a shadow even smaller than itself, showing that light spreads out beyond it.

It was also once thought that light didn’t show the characteristic of waves called interference. However, an English scientist named Thomas Young performed a conclusive experiment in 1801 called the “double-slit” experiment, which showed that light can be made to exhibit both diffraction and interference. Finding no other way to interpret the results of the double-slit experiment, the skeptical scientists were forced to accept the wave-nature of light.

The story doesn’t end there, because in 1905, a 26-year-old patent clerk published a paper arguing that the photoelectric effect, which occurs when light shines on a piece of metal and causes electrons to be ejected, can only be fully understood if you think of light as being made up of particles. Finding no better interpretation for the photoelectric effect, scientists eventually accepted the wave-particle duality of nature, meaning that light has both wave-like and particle-like properties. The young patent clerk received the Nobel Prize for Physics. His name? Albert Einstein.